Conventionally, a capacitor made of a plastic film having a metal deposited thereon (hereinafter referred to as a “metallized film capacitor”) has widely been used. Such films include a polypropylene film, polyethylene terephthalate film, polyphenylene terephthalate film, and polyethylene naphthalate film. Particularly, a metallized film capacitor using a polypropylene (PP) film finds a wide range of applications, from small electronic components represented by portable devices to large-scale industrial use, e.g. drive motor control of a train and high-voltage power capacitor. This is because the metallized film capacitor using a PP film has excellent electrical characteristics, e.g. a small dielectric loss, high withstand voltage, and little fluctuation of the dielectric constant caused by changes in temperature and frequency. Additionally, the metallized film capacitor using a PP film is relatively inexpensive.
FIG. 4 is a sectional view of a conventional metallized film capacitor using a PP film. The capacitor comprises two sheets of PP film 11 and metal-sprayed portions 30. Each of the two sheets of PP film 11 has metal 12 deposited on one side thereof, and are overlaid and wound or laminated. Metal-sprayed portions 30 are formed by spraying metal from both edge faces of the capacitor.
Widely employed structure of deposited metal 12 is a heavy edge structure shown in FIG. 4. Deposited metal 12 in a capacity-forming electrodes portion is thinner to improve self-healing capability and the metal contact portions with sprayed metal 13 and 14 in contact with metal-sprayed portions 30 are thicker to enhance the contact strength with respect to metal-sprayed portions 30. The self-healing capability enables restoring of the capability of the capacitor when local dielectric breakdown has occurred in a portion of a film. Evaporation and scatter of the deposited metal around of the portion interrupts the current. Commonly used as deposited metal 12 is aluminum, zinc, or mixtures thereof. When aluminum is used, its bonding strength wih respect to metal-sprayed portions 30 is weak. Additionally, when alternating voltage is applied to the capacitor for an extended period of time, deterioration of aluminum caused by oxidation thereof decreases the capacity. Because of these problems, in recent years, deposited metal essentially consisting of zinc is used in many cases. Deposited metal made of mixtures of zinc and aluminum is also used to improve humidity resistance of zinc.
In a structure shown in FIG. 4 (hereinafter referred to as a “one-side metallized film capacitor”), two sheets of PP film each having deposited metal on one side thereof are used. Thus, each sheet requires a vacuum deposition step and this necessitates a large number of man-hours. If metal can be deposited onto both sides of a PP film (hereinafter referred to as a “both-side metallized PP film”) by one vacuum deposition step and the film can be placed on a non-metalized polypropylene film having no deposited layer (hereinafter referred to as a “non-metallized PP film”), the vacuum deposition step can be saved to one half.
However, a conventional both-side metallized film capacitor has the following problems. Thus, its performance is inferior to that of a one-side metallized film capacitor.
First problem: when the conventional both-side metallized film capacitor is charged at low temperature or room temperature for an extended period of time, partial discharge occurs in an air gap between the films and deterioration develops. For a one-side metallized film capacitor, voltage is applied to each of the films via one layer of air gap. In contrast, for the both-side metallized film capacitor, the both-side metallized PP film has no air gap; however, the non-metallized PP film has air gaps on both sides. Thus, deterioration caused by partial discharge develops on both sides of the non-metallize PP film. Therefore, the deterioration develops at low temperature or room temperature at which partial discharge is prone to occur.
Second problem: the conventional both-side metallized film capacitor has weak contact with metal-sprayed portions. In general, for metallized film capacitors, heat aging is performed to complete the beat shrinkage of the film after metal-sprayed portions are formed at both edge faces of the capacitor element. This heat aging process is performed at high temperature to alleviate the residual stress and strain produced when the film is drawn lengthwise and widthwise in a two-way drawing step and to stabilize the dimension of the film by heat shrinkage.
When the film is shrunken widthwise in the heat aging process, stress occurs between metal-sprayed portion 30 and contact portion of deposited electrode 13. However, for the one-side metallized film capacitor of FIG. 4, each of the films is in contact with the metal-sprayed portion only at one edge and thus the stress is extremely small. In contrast, for the both-side metallized film capacitor, the both-side metallized film is in contact with the metal-sprayed portions at both edges. Thus, large stress occurs at both edges and the contact portions are prone to deteriorate. Especially, heat shrinkage of polypropylene (PP) film is larger than that of a polyethylene terephthalate film, polyphenylene terephthalate film, and polyethylene naphthalate film. Thus, a PP film is prone to cause contact failure in metal-sprayed portions.
Third problem: in the case of a capacitor element impregnated or filled with insulating oil 70, the insulating oil permeating into a both-side metallized film swells the film. Thus cracking or peeling of the deposited metal occurs and the resistance of the deposited layer increases. Especially, a PP film has an adhesive strength with respect to the deposited layer smaller than that of other films and insulating oil is more permeable into PP films. Therefore, increase in the resistance of the deposited layer deteriorates tan δ and deterioration of the deposited layers in the vicinity of metal-sprayed portions deteriorates contact.
Fourth problem: the conventional both-side metallized film capacitor is prone to cause insulation breakdown when over voltage is applied thereto. In order to improve adhesion of a PP film to a deposited layer, it is common that wettability of the surface of the PP film is improved by corona treatment in production of the PP film. In general, in the corona treatment, a drawn film runs in corona discharge generated at high voltages ranging from several thousands to several dozens of thousands of volts so that the surface of the film is activated. However, a PP film having a small thickness of approx. several micrometers for use in a capacitor is damaged by the corona treatment and thus dielectric strength thereof decreases. The PP film for use in a capacitor has a large number of projections and depressions. When both sides of such a film are metallized, the thickness distribution of the film has direct influence on the dielectric strength of the capacitor. Therefore, because a portion of the film having depressions on both sides is thin, the dielectric strength of the capacitor decreases. On the other hand, for a one-side metallized film capacitor, even if the film has depressions, always existing air gaps alleviate the voltage applied to the films. For this reason, the dielectric strength of the one-side metallized film capacitor is higher than that of the both-side metallized film capacitor.
Fifth problem: in production of a both-side metallized film, the deposited layers are peeled by blocking. After deposited layers are formed on both sides of the film in the vacuum deposition step, the both-side metallized film is wound once in a vacuum evaporator. At this time, the deposited layers on both sides are in contact with each other. A PP film has low adhesive strength with respect to a deposited metal. Additionally, the roll formed after vacuum deposition is tightly contracted (i.e. the stress accumulating inside of the roll of the both-side metallized PP film is large). Thus, deposited metal layers on both sides adhere to and peel from each other. This phenomenon is called blocking. It is known that the blocking is remarkable with a deposited metal of zinc.
To address this problem, trials of decreasing blocking when zinc is deposited on both sides of a PP film have conventionally been made. For example, U.S. Pat. No. 3,895,129 discloses a method of depositing zinc on both sides of a PP film. In this invention, after the deposited surface on one side is air-sprayed to oxidize the deposited metal surface, the film is wound into a product roll. The Japanese Patent Examined Publication No. H07-24251 discloses a both-side metallized polyolefin film capacitor having a both-side metallized polyolefin film and a film for combination. For the both-side metallized polyolefin film, zinc is deposited on both sides of a polyolefin film that contains oxidation inhibitor having a melting point of at least 156° C. and an aluminum layer is formed on the deposited metal.
However, even in the case using the method disclosed in U.S. Pat. No. 3,895,129, blocking is not completely eliminated. It is known that observations with an optical microscope and the like show the existence of a small peeled area of the deposited layer in some cases.
In other words, because blocking is caused by the contraction of a roll of both-side metallized PP film as described above, the deposited layer may peel in the inner circumference of the roll where the stress caused by the contraction accumulates even when no blocking is seen in the outer circumference of the roll. Especially increasing the tension on the PP film at vacuum deposition increases the contraction. Thus, blocking in the inner circumference is prone to increase.
Further, because blocking is a phenomenon in which deposited layers adhere to and peel from each other during unwinding, it is known that the adhesion of the deposited layers develops and the peeled area increases when the film is kept for an extended period of time in the state of the roll.
When a capacitor is produced using a both-side metallized PP film in which blocking occurs, peeling of the deposited electrodes not only decreases the effective area but also affects the electrical characteristics as a capacitor in some cases. Especially in the heavy edge structure having thicker electrodes in the metal contact portions with sprayed metal, the deposited electrodes in the capacity-forming electrodes portion are thin. For this reason, even when a small area of the electrodes peels, the resistance of the deposited electrodes significantly increases and the capability of the electrodes is lost. Therefore, tan δ (dielectric loss) increases and the characteristics inherent to a metallized PP capacitor, i.e. a small loss, cannot be obtained.
Further, when zinc is deposited on both sides of a PP film and an aluminum layer is formed on the deposited metal as described in the Japanese Patent Unexamined Publication No. H07-24251, zinc-deposited layers are not in contact with each other and thus blocking is prevented. However, because the aluminum layer extends to cover the electrode leads, the feature of zinc vacuum deposition, i.e. excellent contact with the metal-sprayed portions, is affected.